A solar cell panel comprises a matrix of solar cells, which convert sun light into electricity. Each solar cell is produced by treating a silicon wafer. Silicon wafers are cut out of a large silicon body called ingot.
The ingot is manufactured in a furnace, and good care is taken to control among other things the ingot's temperature during manufacturing, since it plays a very important role in defining the quality of the ingot.
A large ingot has normally an uneven quality. This uneven quality is reflected in the wafers cut from the ingot and in the cells, and it will lie as a bottom line governing the wafer and cell characteristics.
Cell quality can be measured swiftly and reliably. Usually cell quality is determined by means of an inspection, where the cell's surface, current, voltage, efficiency and shunt resistance are measured. Ingot and wafer quality can not be measured as precisely as cell quality, and a full characterization of wafer and ingot quality always includes cell processing of the materials. Detailed wafer and ingot quality measurements are very costly and require several weeks of testing, which makes them unsuitable for a production process.
The invention has thus as an object to provide a method for establishing correspondence between a cell and a wafer, that is, identifying which cell is produced from which wafer.
By means of the invention cell quality will easily be correlated to wafer and ingot quality and the furnace and other production parameters can be modified to increase the electrical quality and mechanical strength of both wafers and cells.
There are several methods for tracing a cell back to a wafer:    1) Wafer scribing/inking: An identification of the wafer is written on the wafer's surface by means of a marker, as described in e.g. U.S. Pat. No. 6,482,661. A plurality of wafers are sliced from the ingot with a portion of the ingot indicia. Wafer indicia is then marked on the peripheral edge of the wafer. Said indicia is read by means of a camera and information is stored. JP 10321690 describes a method where a wafer number is printed in the peripheral part of a surface where a pattern is not formed. The wafer number is read by means of a CCD camera and recognized by a recognizing part. These methods have the disadvantage that they lead to reduction in the wafer's surface quality. They are besides not suitable for mass production.    2) A small cut is performed on the wafer. This method leads to increased breakage ratio.    3) Tracking by means of advanced data systems (e.g. as described in U.S. Pat. No. 6,330,971) which “follow” a wafer during all the process. This alternative is highly expensive as it requires adapting the tracking system to equipment produced by different entities.
Alternatives 1) and 2) are presently only used in the field of research. Alternative 3) is possibly used for very high amounts of cells, where the size of the batches justifies a high investment in tracking system, but to the applicant's knowledge this is not used in mass production today.